updatedb is for updating locates database, which is a search program. It will let you search for things like filenames that you think might be missing, etc...
you do not need libs in your path, only commands that you are going to issue
path is like a search path for your shell
$ echo $PATH
you issue a comand like
if it is not in the first directory in your path then the next directory in the path will be checked, and so on untill the command is found in /usr/X11R6/bin
if that directory was not in the path you will get command not found
libs have ld.so.cache